It is well understood in the art that precise time-of-arrival position determination is dependant upon the accuracy of the transmitter clocks used. In its most rudimentary form, three transmitter beacons positioned at known locations and connected to a common clock via three identical length cables will suffice as the basis for a time-of-arrival positioning system. However this rudimentary positioning system is highly impractical to manufacture and install due to the requirement for precisely timed cables distributing high frequency timing signals over potentially large distances between beacons. Alternatively, precision atomic time standards, which have very low drift rates, may be installed at each transmitter beacon and monitored using a reference receiver positioned at a known location and connected to a reference timebase. In response to positioning signals received from the transmitter beacons, clock corrections are sent from the reference receiver via an RF data link to each beacon, for subsequent retransmission to user equipment. Modern satellite positioning technologies such as GPS employ this technique, wherein cesium and rubidium time standards are installed in each GPS satellite, with the GPS Ground Control Segment continually monitoring all GPS satellites and up-linking clock corrections to each satellite every twenty four hours. These corrections are then rebroadcast via each satellite's navigation message to GPS user equipment, so that positioning algorithms within the GPS user equipment can account for satellite clock error. With at least four GPS satellites in view, a three-dimensional position is accomplished in GPS user equipment using a standard technique known as a conventional code-based GPS position solution. This standard technique is also generally termed “a single point position” by those skilled in the art.
Conventional Code-Based GPS Position Solution (Single Point Position)
In conventional code-based GPS, the latitude, longitude, and altitude of any point close to the earth can be calculated from the propagation times of the positioning signals from at least four GPS satellites in view. A GPS receiver makes range computations based on the correlation of internally generated pseudorandom code (PRN) sequences with received pseudorandom code sequences from each GPS satellite. The measured ranges are referred to as pseudoranges as there is a time difference, or offset, between the clocks on the satellites and the clock within the GPS receiver. It is necessary to ensure that the receiver's clock is synchronized with the satellite constellation's clock in order to accurately measure the elapsed time between a satellite's pseudorandom code sequence transmission and reception of that pseudorandom code sequence by a GPS receiver. A navigation message is also transmitted from each satellite, which includes time information, satellite orbital information, and satellite clock correction terms. For three-dimensional positioning a GPS receiver requires four satellite signals to solve for the four unknowns of position (x, y, z) and time (t). For two-dimensional (2-D) positioning, altitude is fixed, and three satellite signals are required to solve for three unknowns of position (x and y) and time (t). A conventional code-based GPS position solution is able to provide a GPS receiver, with at least four satellites in view, the capability to determine an absolute three-dimensional (3-D) position with an accuracy of approximately 10 to 20 meters.
This Conventional Code-based GPS position solution is an autonomous solution, which can determine position, velocity, and time (PVT) without differential correction data from reference receivers. It has therefore become known as a “single point” position solution in the art.
Conventional Code-Based Differential GPS (Relative Positioning)
With an established accurate atomic timebase the GPS constellation is only capable of providing a GPS receiver with an absolute three-dimensional position accuracy of approximately 10 to 20 meters. This is due to the corruption of positioning signals from six major error sources: (1) ionospheric delay, (2) tropospheric delay, (3) ephemeris error, (4) satellite clock error, (5) GPS receiver noise and, (6) multipath. Ionospheric delay is the varying time delay experienced by electromagnetic waves when passing through bands of ionized particles in the ionosphere. Tropospheric delay is the time delay experienced by electromagnetic waves when passing through moisture in the lower atmosphere. Ephemeris error is the difference between the actual satellite location and the position predicted by satellite orbital data. Receiver noise is the noise generated by the internal electronics of a GPS receiver. Multipath is the signal delay caused by localized signal reflections in close proximity to a GPS receiver. The majority of these error sources are spatially correlated over relatively short distances (i.e. tens of kilometers). This means that two different GPS receivers within this proximity to one another will observe the same errors. It is therefore possible to improve the spatially correlated error sources using a technique known as “Differential Correction”. A reference receiver placed at a well-known location computes an assumed pseudorange for each satellite signal it detects. It then measures the received pseudoranges from the GPS satellites and subtracts the assumed pseudoranges from the received pseudoranges, forming a differential range correction for each satellite in view. The reference receiver then sends these corrections as digital data to the GPS receiver via an RF data link. The GPS receiver subsequently adds these corrections to the pseudoranges it measures (for the same satellites in view to the reference receiver) before calculating a position solution. Errors common to both reference receiver and the GPS receiver are completely removed by this procedure. Uncorrelated error sources such as multipath and receiver noise remain in the pseudoranges and subsequently degrade position accuracy. Position accuracies in the order of several meters are achievable with code-based differential GPS correction in low multipath environments.
Conventional Carrier-Based Differential GPS (Relative Positioning)
Conventional carrier-based differential GPS (CDGPS) calculates the difference between the reference location and the user location using the differences between the carrier phases of the satellites measured at the reference receiver and the user receiver. A CDGPS reference receiver, installed at a well-known location, calculates simultaneous carrier phase measurements for all satellites in view, and then broadcasts carrier phase data to the user receiver via an RF data link. The user receiver also calculates simultaneous phase measurements for all satellites in view, and subsequently computes a phase difference to determine the position of the user receiver with respect to the reference receiver location. The carrier phase measurements are a running cycle count based on the Doppler frequency shift present on the carrier frequencies from the GPS satellites. Each epoch, this running cycle count (the value from the previous epoch plus the advance in phase during the present epoch) is available from the receiver. More specifically, the advance in carrier phase during an epoch is determined by integrating the carrier Doppler offset over the interval of the epoch, hence the name Integrated Carrier Phase (ICP).
The user receiver can measure the fractional phase plus an arbitrary number of whole cycles of the carrier, but cannot directly determine the exact number of whole cycles in the pseudorange. This number, known as the “integer cycle ambiguity”, must be determined by other means. Traditional strategies for resolving carrier phase integer ambiguities fall into three broad classes: search methods, filtering methods, and geometrical methods. These traditional methods do not yield instantaneous integer cycle ambiguity resolution. A technique, known as “wide-laning”, has been developed to overcome the non-instantaneous integer cycle ambiguity problem. Wide-laning multiplies and filters two carrier frequencies (traditionally the GPS L1 and L2 frequencies) to form a beat frequency signal. This beat frequency wavelength is significantly longer than the wavelengths of the two individual carriers. Consequently, resolution of the integers can be accomplished by using pseudorange observations to determine the integer ambiguity of the wider “lanes” formed by the beat frequency signal. These, in turn, greatly reduce the volume of integers that must be searched to resolve the integer ambiguity.
The main constraints for CDGPS methods are firstly the integrity and latency of the RF data link, and, secondly, the lack of time determination at the user receiver. The data bandwidth of the RF data link constrains differential data update rates, causing data latency and degrading position accuracy. Poor reception of differential data caused by physical obstruction and multipath causes data corruption, which degrades position accuracy at best, and results in total link failure and no position update at worst. The second shortcoming of CDGPS is the lack of time determination. A conventional single point position solution solves for the four unknowns of position (x, y, z) and time (t). CDGPS uses a process known as “double differences”, which eliminates the receiver clock terms for both the reference receiver and the user receiver. Therefore, the user receiver can determine accurate position with respect to the reference receiver position, but cannot determine time. This is unimportant if the user is simply, and only, interested in position. However, precise knowledge of an accurate system timebase is very beneficial to many user applications involving computer networks and telecommunication systems. The lack of time determination is a major problem associated with CDGPS prior art systems.
Pseudolite Augmentation
Another approach used to aid GPS position determination is the use of ground-based augmentation systems such as pseudolites. Pseudolites can be incorporated into Conventional Code and Carrier-based Differential GPS systems without any additional infrastructure requirements. They can be used as additional ranging signals, and also as RF data links to send differential corrections to user equipment. Alternatively, pseudolites can be synchronized to the GPS timebase. A GPS receiver determines GPS time from a conventional code-based GPS solution using at least four GPS satellites and passes the determined time to a co-located pseudolite transmitter. The accuracy of the GPS timebase is constrained by GPS error sources including ionospheric and tropospheric delay, satellite clock error, satellite position error, receiver noise, and multipath. Time accuracies of approximately 50 to 100 nanoseconds are achievable by using the GPS timebase method, however this translates to position accuracies only in the order of tens of meters. This accuracy is much too coarse for precise navigation systems.
Carrier-Based Differential GPS Using an “Omni-Marker” Pseudolite
U.S. Pat. No. 5,583,513 to Cohen, titled “System and Method for Generating Precise Code-based and Carrier Phase Position Determinations” describes a differential correction method whereby a so called “omni-marker” pseudolite serves as a channel for relaying information to a position receiver for making differential ranging corrections (Column 6, lines 43 to 46). The omni-marker pseudolite can be described as a metaphorical mirror, whereby GPS satellite signals are “reflected” in-phase from the known omni-marker pseudolite position to the position receiver. Thus, the out-going carrier and PRN code components of each of the beacon marker signals is exactly phase coherent with respect to their incoming counterparts in the GPS signals (Column 6, lines 28 to 32). A position receiver situated in an over-flying aircraft receives positioning signals from the GPS satellites and also receives “reflected” GPS positioning signals from the omni-marker pseudolite, and subsequently computes differential range measurements.
Cohen's differential method eliminates the need for a traditional digital data link, as required by conventional code and carrier-based differential systems. However, an omni-marker position receiver must still receive both GPS satellites and omni-marker signals to compute a differential range measurement. Receiving omni-marker signals alone will not allow a position computation. Also, the omni-marker must generate and transmit individual carrier and PRN components for each GPS satellite in view, making the omni-marker complex and expensive. Currently, this would require up to twelve individual transmissions from a single omni-marker. Further, an omni-marker position receiver requires double the receive channels of a conventional differential GPS receiver, adding to the cost and complexity.
Differential Range Measurements Using “Ground Transceiver” Pseudolites
U.S. Pat. No. 6,121,928 to Sheynblat, titled “Network of Ground Transceivers” describes a differential correction method whereby a network of so called “ground transmitter” and “ground transceiver” pseudolites serve as channels for relaying information to a position receiver for the differential determination of user position (Column 5, lines 31 to 36). Sheynblat teaches the use of differential correction to overcome master clock bias (Column 5, lines 23 to 36) and line biases introduced by the ground transceiver hardware (Column 5, lines 38 to 67 and Column 6, lines 1 to 23). Sheynblat's differential methodologies and embodiments include: (i) a user receiver differencing ground transceiver signals with a ground transmitter signal (Column 5, lines 31 to 36, and claim 2), (ii) a user receiver differencing multiple master ground transmitter signals with a ground transceiver (Column 6, lines 25 to 67, Column 7, lines 1 to 33), and (iii) a user receiver differencing ground transceiver signals, which incorporate signals that have been differenced with a satellite signal (Column 1, lines 34 to 67, Column 8, lines 1 to 34). Sheynblat's patent teaches an advance of differential methodologies but does not teach, show, or suggest a highly desirable system that would produce single point position solutions in a roving position receiver from a network of ground transceivers.
Prior art systems will not allow time-of-arrival position determination without requiring at least one of: (a) a physical connection between transmitter beacons; (b) an atomic time standard at each transmitter; (c) synchronization to a GPS timebase; or (d) some form of differential correction. A system that can provide extremely precise time-of-arrival positioning signals, without any of these constraints, is highly desirable. The present invention achieves this desirable goal by chronologically synchronizing a system of transceivers (hereafter referred to as a Positioning-Unit Devices), as described below.